But as i have learned, Lord Buddha's understanding is above any other,even above an arahant.

Maybe. But I haven't met him, and I'm inclined to believe none of us have. So I can't really put his word (which word? where?) above any other even if I wanted to.

Dhamma/Dharma itself is timeless and therefore does not need modifications or updates.

There are proposals for clarifications and emphasis -- people just disagree with them and each other.

So what is the need of the term "Modern practice / Modern Buddhism"?

Times change, methods change, pedagogy changes. The subject at hand might still be the same, what one ends up learning might still be the same, but the ways of teaching them always change with cultural and geographical developments.

Shouldn't we take Lord Buddha's word above the rest and use any other resource later on for further clarification if there is such a need?

Some people take mainly the pali suttas to be his words, other's take mainly their master's teachings as his word. Could the pali contain misrepresentation of his words? Could a master misrepresent him? If so, what is the Buddha's word?

Why there is a trend of drifting away from original teachings, does the followers now believe that Lord Buddha's words are outdated?

What original teachings? Hundreds of years after his death, different regions across asia ended up developing different canons. There are remarkable overlappings, but there are also some disagreements. And within agreements in words, there are disagreements in interpretation. What is the original interpretation?

Some literal teachings could be deemed outdated, yes. For example, westerns have a very hard time sitting cross-legged. Should sitting in that position be paramount for buddhist practice just because a text have the Buddha mentioning that one sits cross-legged when practicing meditation?

Other teachings could be seen as figurative and unfitting for a "modern mind": rebirth is a favorite one here.

There's always a trend of drifting away of anything. One (buddhist) answer is because that's what a wordling mind does: it drifts away.

But as i have learned, Lord Buddha's understanding is above any other,even above an arahant.

Maybe. But I haven't met him, and I'm inclined to believe none of us have. So I can't really put his word (which word? where?) above any other even if I wanted to.

Dhamma/Dharma itself is timeless and therefore does not need modifications or updates.

There are proposals for clarifications and emphasis -- people just disagree with them and each other.

So what is the need of the term "Modern practice / Modern Buddhism"?

Times change, methods change, pedagogy changes. The subject at hand might still be the same, what one ends up learning might still be the same, but the ways of teaching them always change with cultural and geographical developments.

Shouldn't we take Lord Buddha's word above the rest and use any other resource later on for further clarification if there is such a need?

Some people take mainly the surtas (e.g. pali) to be his words, others take mainly their master's teachings as his word, and so on. Could the sutras contain misrepresentation of his words? Could a master misrepresent him? If so, what is the Buddha's word?

Why there is a trend of drifting away from original teachings, does the followers now believe that Lord Buddha's words are outdated?

What original teachings? Hundreds of years after his death, different regions across asia ended up developing different canons. There are remarkable overlappings, but there are also some disagreements. And within agreements in words, there are disagreements in interpretation. What is the original interpretation?

Some literal teachings could be deemed outdated, yes. For example, westerns have a very hard time sitting cross-legged. Should sitting in that position be paramount for buddhist practice just because a text have the Buddha mentioning that one sits cross-legged when practicing meditation?

Other teachings could be seen as figurative and unfitting for a "modern mind": rebirth is a favorite one here.

There's always a trend of drifting away of anything. One (buddhist) answer is because that's what a wordling mind does: it drifts away.

But in a gist, those who bring something new (or modern) in contrast to a set of texts are either doing because:

1. they regard it as compatible and the same with what the texts teach
2. they challenge the authenticity of the text